Fix IQM root joint backlerp when joint number is more than 0

Backlerp for root joint (joint with no parent) was always applied to
joint 0. This is an issue when a model has multiple root joints.

code/renderergl1/tr_model_iqm.c

@@ -983,7 +983,7 @@ static void ComputePoseMats( iqmData_t *data, int frame, int oldframe,
 				
 			} else {
 				InterpolateMatrix( mat1 + 12*i, mat2 + 12*i,
-						   backlerp, mat );
+						   backlerp, mat + 12*i );
 			}
 		}
 	}

code/renderergl2/tr_model_iqm.c

@@ -987,7 +987,7 @@ static void ComputePoseMats( iqmData_t *data, int frame, int oldframe,
 				
 			} else {
 				InterpolateMatrix( mat1 + 12*i, mat2 + 12*i,
-						   backlerp, mat );
+						   backlerp, mat + 12*i );
 			}
 		}
 	}